. >> the judge in the murder trial of george zimmerman said prosecutor audio experts cannot testify about the screams heard on the calls to 911. zimmerman is of course the accused vigilante charged with killing an unarmed teenager in florida. the tapes are a key piece of evidence in the trial which is set to begin tomorrow morning. >> it's considered one of the most important and controversial pieces of evidence from the night former community watchman george zimmerman shot and killed 17-year-old trayvon martin. >> you think he's yelling help? >> yes. >> all right, what is your -- >> there's gunshots. >> martin's parents have said their son can be heard screaming for help in the background of the 911 call. zimmerman's father has countered it's his son calling out during a confrontation that night. the last word in the role the call will play in the second degree murder trial was issued by judge debra nelson this weekend. the recording will be allowed to be entered as evidence in the case but in a written ruling issued she said the jury will not hear from state audio experts who have sugge